I'll attempt to keep this guide as spoiler-free as possible. I won't be explaining why one would want to spend entirely too much time and effort to reach the top of the NSC Tower - there are plenty of detailed guides regarding that. However, all of those guides mention that Levitate is required for the step involving the NSC Tower. I'm here to describe how one can complete this step without Levitate.

Obviously, you can simply choose to wait until you obtain Levitate to complete this. However, if you're like me, you read about what your reward is and thought "Well, that seems like it would be nice to have NOW, not LATER". If that's the case, read on.

NOTE: I didn't know exactly how to get to my destination, or if it was even possible to do so without Levitate. However, I wanted to try, and I ended up not wanting to give up once I started getting closer. With that said, I ended up spending a total of ~6 hours attempting this (and failing entirely too many times because I'm bad), and didn't think to take screenshots of some of the later steps of the process. I'll explain as clearly as I can, I may go through the process again and take more screenshots if there's a demand for them. But, I make no guarantees of how long this might take the average person, or if you will even want to try.

A few things to note before we get started:

• The room will fully or partially reset should you approach the outer edge of the room or fall all the way back to the bottom floor. This can be helpful if mistakes are made, but is annoying (or problematic) if you're in progress and have NOT made a mistake. Keep this in mind as you go through this process.
  
  
• This guide relies entirely on (ab)using Dodge/Evade, character movement (climbing objects) and physics. It may take you many tries to successfully complete this.

In relation to the above two points, a few suggestions/hints that I learned along the way to make your life easier:

• If you "lose" a critical object for any reason (detailed in the later steps), your easiest method of recovery is to simply go back to the outer rim of the room (thus resetting the objects in question) and start over from that point. Dying (and thus losing Source) is not required.
  
  
• This one may seem obvious, but... Make every effort not to die or fall all the way to the floor if you miss a jump. Doing either one will reset basically everything important in this guide, which makes this a VERY time consuming process. Trust me, I know...
  
  
• Equip player mods that either reduce the Energy cost of Evade, or boost your maximum Energy (or both). You may end up using Evade 5-6 times before touching the ground to recover Energy. This is primarily in the (somewhat likely) event that you miss a jump and need to "recover" by Evading back onto the tower to avoid a room reset and/or death.

If you're still early in the game (prior to being sent on your way to Black Rock Quarry), start here. If you've already been sent there and have unlocked the "NSC Power Plant Upper Level" Control Point, you can save yourself some time by fast traveling there and skipping to the next section of this guide.

Fast travel to the "NSC Power Plant" Control Point. From the Control Point, face the NSC Tower. Follow the sign to the right and stop at the forklift under the bundle of large black cables:



Climb on top of the forklift, then jump across to the platform with the pipe. You may need to use Evade to reach it.

Once on the platform, jump across to the bundle of black cables. You will likely need to use Evade at least twice to make it:



Once you're safely on top of the cables, face the tower and run up the cables. Use Evade again to make the jump to the platform.

If you started from the bottom of the tower, make your way all the way up to the Black Rock Quarry level and cleanse the "NSC Power Plant Upper Level" Control Point so you respawn there should you meet an untimely demise.

From the upper Control Point, our first stop is two levels back down the tower, to the right of the catwalk, directly in front of the stairs. You can either follow the catwalks back down, or simply jump and Evade onto the second level down. Your destination is here:



Grab that helpfully-labeled square base, and bring it with you back to the upper Control Point level. You'll likely need to drop it here, climb up to the catwalk and then grab it again:



You'll need four of these bases in total, the other three are already on the upper level. All three are along the tower walls, two are stacked directly in front of the catwalk leading to the Control Point, the third is partway around the tower.

Launch all four bases up to the next level of the tower. You may need to experiment a bit, but I'm aiming right about where you want to be aiming in this screenshot:



You'll want to stand well back from the tower, but not so far back that the room resets (refer to the previous hints). If you stand too close, the base will end up bouncing off the wall (or floor, or another base, or the railing...) and falling into the abyss at the bottom of the room, requiring you to reset the room and start over unless you manage to catch it again in mid-air. The railing on this level (and the others above it) cannot be destroyed by throwing objects at them, and the square bases do like to bounce.

Alternatively, you can attempt to stack all of the bases on the catwalk so they're accessible from the next level and pull them all to you once you reach it. I had better luck Launching them from below, but your mileage may vary.

Once all four bases are on the next level (or you've made them accessible from there), you're ready for the first challenge.

Note that Hiss will semi-frequently (re)spawn on either of the upper catwalks. This makes for good farming of resources, but does get annoying if you're making your 200th attempt.

Using any objects available nearby OTHER than the previously mentioned square bases (without going close to the edge of the room and thus resetting it), clear off the railing on the right side of the catwalk:



The bodies of the previously mentioned Hiss also work for this task, if they're available.

Make a running jump from the catwalk onto the silver pipe on the right. This jump requires one use of Evade, and is probably the most difficult part of this entire process. You'll need to land in just the right position - either directly in the center, or in the position to force a "climb" onto the pipe. Your goal is to hit the bottom of the slant and then run/jump/climb to the flat at the top. You'll need to be nearly perfectly centered in order to not fall off on your way up.

Note from Fantastic Mr. Spock in the comments below: That first jump to the single pipe is much easier if you stack two barrels, or one of the orange cylinders and jump off from there - Don't ask why I didn't think of this. It hurts.

Once you're on the flat, you'll basically want to repeat the process over to the set of two pipes on the other side of the catwalk:



This jump will require 2-3 uses of Evade as quickly as possible in order to not miss the pipes, but is somewhat easier than the first jump, since there's two pipes to choose from. Again, you'll need to be almost perfectly centered on either of the pipes in order to run/jump/climb to the flat at the top. Either pipe works, but you likely cannot make it to the top from between the two pipes, so pick one.

Once you're on the flat, jump from the pipe to the railing and climb over. I found this to be slightly easier from the left pipe jumping to the left, but your mileage may vary.

Once you make it to the first level, carefully stack your four square bases below the single silver pipe (the first one you jumped to in the previous section), set off to one side. You'll need to make a "leaning tower" so to speak, so that you can climb each level and stack the next level on the level you're standing on. It helps to start towards the middle of the platform and build towards the wall, so the wall supports the tower.

Once your "ladder" is built, you should be able to jump from the square bases onto the single pipe. You may need to force the top base on the stack to land on it's side to get enough height. Unfortunately, this is the part I didn't think to take screenshots of and I don't recall if that was necessary or not.

Once you're on the pipe, jump/Evade across to the set of double pipes.

Once on top of the double pipes, carefully pull the top base of your stack up to you. Get as close to the edge of the pipes as you can so there's room to drop (not throw) the base at your feet on top of the pipes. This may take quite a bit of work as there isn't much room for the base under the platform. However, you need the base as close to the edge as possible for the next step.

Once the base is at your feet, climb on top of the base, and then climb over the railing onto the next level.

From the next level, pull the base you just used up to you. You may need to jump in order to do so, depending on how close to the edge you were able to get it. Just be (very) careful not to actually jump over the railing.

Depending on your skill/confidence level and exactly where the rest of your stack of bases is, you'll either need to repeat these steps (drop back down to the double pipes - NOT the next floor down - and pull the base up to you, etc) or pull the remaining three directly up to the next level. If you need to repeat the steps, just be mindful of the fact that if you fall off the double pipes and/or lose more than one of the bases, you'll need to reset the room and start over. You need all four bases to get to this point, and one of them is (hopefully) now inaccessible on the next level up.

You need a MINIMUM of three bases to continue beyond this point. Four is ideal. I managed with three after losing one over the edge into the abyss, but it would have been quite a bit easier with all four. If you lose more than one, reset the room and start over. There is ONE object on the next level that can be used as a substitute for the missing base, but it's not as easy to maneuver.

From this level, you basically want to repeat your previous "ladder" in order to climb up to the next railing. If you're missing one base, stack the object found on this level at the top of your ladder. There is a spool of hose that can be used to support the leaning stack far enough away from the wall.

Once you're on the next level, starting at the top of your stack, pull your bases (and the other object if you're missing a base) up to you and make a stack that can be used to climb onto the large brown pipe.

Once on the pipe, you'll need to be EXTREMELY careful with the remaining steps.

Starting at the top of your stack, pull the bases/object up to you. If you're using the non-base object, set it to the side (but still on the pipe) so you can put it at the top of the stack again. Otherwise, carefully place the bases on the pipe so they're balanced on top of the pipe, repeating the "ladder" one more time.

It's quite difficult at this point to stack the bases on top of each other without the one you're holding pushing the stack off of the pipe, so again, be very careful. Eventually, you should end up with something resembling this:



Allow 10-20 seconds after placing the top object for the stack to "stabilize", so your attempt to jump on top doesn't result in the whole stack falling over. Once it's stable, you should be approximately here:



Jump (Evade if necessary) to the railing and hopefully climb over it. If you were successful...

... What, you want more? If you made it this far, you know why you went through this nightmare. Take your hard-earned loot where it needs to go. I'd recommend jump-Evading down one or two levels at a time to be safe. Keep in mind the Hiss on the catwalks will likely respawn on your way down if they haven't already, and they are very trigger happy with their rockets.

Now, go enjoy a drink of your choice. If you actually made it through this, you deserve it.